The Covalent Binding of C3 and C4 to Cell Membranes - The labile binding site of C3b interacts with receptive surfaces by way of a covalent bond. A study of the kinetics of release of bound C3b by hydroxylamine suggests that the bond between the complement protein and receptive surfaces (RS) is an ester linkage having the form RS-O-CO-C3b. C4b also binds to all surface components by an ester linkage. C3 hemolytic and covalent binding activities decline with identical kinetics demonstrating a direct correlation between the two. We conclude that covalently surface-bound C3b is the biologically active form of this complement protein. Studies of the inactivation of C3 by 14C-methylamine show that the inactivation corresponds quantitatively with the labeling of C3 in the C3d domain. These studies also support a hypothesis for an internal thioester within C3 binds to receptive surfaces by transfer of the acyl function of the thioester to a hydroxyl group on the receptive surface. This proposed model for the reaction of C3 with receptive surfaces also applies to C4. Our goal for the coming year is to understand fully the chemistry of covalent bond formation between C3b and C4b and receptive surfaces.